Link metrics, often referred to as link weights, are sometimes advertised and used in a communications network by devices trying to make routing decisions with regard to which path to use. For example, in the case of shortest path first routing, link metrics indicative of path length are advertised and routing decisions are made based on the advertised link metrics, e.g., with the links which result in the lowest cumulative link metric between a source device and destination device being selected for inclusion in a route between the source and destination device. In other cases where the link metrics represent cost metrics, the advertised link metrics are normally indicative of a cost associated with a link. In a cost based approach to routing a device will normally select the lowest cost path as indicated by selecting a path with the lowest overall cost as indicated by the cumulative sum of the link metrics of each link in the selected path.
Link aggregation allows multiple links, e.g., Ethernet links, into a single logical link between two networked devices. An aggregation of links is often referred to as a bundle. For purposes of advertising a link metric, the logical link is treated as a single link even though it represents a bundle of individual links. Accordingly, a single link metric is normally advertised for a bundle.
Link Aggregation Control Protocol (LACP) is an IEEE standard defined in IEEE 802.3ad. LACP lets devices send Link Aggregation Control Protocol Data Units (LACPDUs) to each other to establish a link aggregation connection. The metric used to facilitate routing decisions is often communicated as a Link Aggregation Control Protocol Data unit.
A useful feature of LACP is that when one member link stops sending LACPDUs (if the cable is unplugged, for example), it is removed from the link aggregation group (LAG). This helps to minimize packet loss.
In the case of link bundles, it is often possible to configure a device to stop advertising a metric for a link bundle if the number of operational links in the bundle falls below a configured number of links, e.g., a number of links expected to be needed to support a normal or anticipated traffic load. Once a metric for the bundle ceases to be advertised, the bundle will normally be removed from the set of links which are considered by devices for routing purposes in accordance with the link protocol being used.
Removing a bundle which can no longer handle the expected normal traffic load reduces the risk of packet loss, e.g., due to overloading of a bundle which is suffering from one or more faulty connections or links which reduce its capacity. Networks are often designed with sufficient redundancy that if one link bundle fails another, e.g., redundant, link bundle will be able to handle the load.
While the removal of one bundle from a network from one or more link failures in the links which form the bundle may not have significant impact on a network, the failure of multiple bundles may result in a portion of a network being coming isolated.
In view of the above discussion it should be appreciated that there is a need for methods and/or apparatus which would allow a bundle to be removed from being used when a functional alternative bundle can serve as an alternative path for the full normal traffic flow but which would leave the bundle as an available link, albeit one that might not be able to support the full normal traffic flow, in cases where failure to use the one or more functional links in the bundle would result in a portion of a network becoming isolated due to the lack of a good alternative link or bundle.